Support loading only approved vendorsetup.sh files

Very few vendorsetup.sh files are needed anymore, since add_lunch_combo
has been deprecated. So add a way so that only approved vendorsetup.sh
files can be loaded into the shell, and others will be skipped.

This further limits the amount of code that can run outside the build
sandbox, and makes this list more visible to tree maintainers before
they're used instead of after.

Test: no allowed-vendorsetup_sh-files
Test: empty allowed-vendorsetup_sh-files
Test: one file in allowed-vendorsetup_sh-files
Test: two files in allowed-vendorsetup_sh-files
Test: non-present file in allowed-vendorsetup_sh-files
Change-Id: Ia23d1c9d11a7295d5be5abd10cf56edbdec80483